FIG. 4 is a diagram for explaining an injection control in a conventional die casting process. The injection control is divided into three steps: a step of charging molten metal at a low speed VI after the molten metal is poured into the plunger sleeve, a step of charging the molten metal into the cavity at a high speed, and a last step of performing an increase in pressure immediately after charging the molten metal into the cavity.
In the high speed charge of molten metal among these steps, a so-called run-around system is adopted which enables pressure oil in the rod side chamber of an injection cylinder to be circulated to the head side chamber, thereby performing a movement of injection at a high speed using a lower quantity of flow of pressure oil.
Further, in the increased pressure control after the charge, pressurizing the molten metal charged into the cavity as soon as possible is indispensable for providing a high quality of flawless casting.
In the prior art, a reference point for calculating a valve switching-over point where a run-around control is switched over to a quick pressure control is taken at a position 0 of starting of the movement in a stroke of the injection cylinder, and the switching-over positon is set to a position at a predetermined distance from the position of limit of the backward movement. A position detector such as a limit switch is provided at the switching-over position, and the control valve is switched over on the basis of a detecting signal from the position detector.
In a die casting process, the existence of dispersion in the swing angles of the ladle often causes a change in a supplied quantity of molten metal. From this reason, in the case where the reference point of switching-over from a high speed control to a quick increased-pressure control is taken to a starting position of the movement of the injection cylinder, the positions where the increase in pressure is actually commenced are different in a relation relative to a high speed injection control.
In this case, when the switching-over operation is performed at a position short of the switching-over position set as an optimum position L, the high speed injection is throttled and a braking action is inconveniently applied.
On the other hand, when the switching-over operation is performed at a position behind the set position, a delay occurs in the increase in pressure, which exerts an influence on the quality of a product.